Chicken leucocytozoonosis is a disease resulting from infection by Leucocytozoon caulleryi belonging to the suborder Haemosporina. L. caulleryi was recognized for the first time in 1909 in Vietnam in the blood of chicken as round gametocytes that do not carry malaria particles, and was named as a protozoan belonging to the genus Leucocytozoon. 
In Japan, chicken leucocytozoonosis was confirmed for the first time in 1954 in Hyogo prefecture. The pathogen was identified as L. caulleryi, and thereafter, experiments demonstrated that this protozoan is biologically transmitted by a type of blood-sucking insect, Culicoides arakawae (hereinafter abbreviated as C. arakawae) belonging to the order Diptera and family Ceratopogonidae. The life cycle of this pathogen has been almost fully elucidated. Leucocytozoon caulleryi has three stages of development: schizogony (multiplicative reproduction) and gametogony (gamete formation) in the body of the chicken host, and sporogony (spore formation) in the body of the C. arakawae transmitter.
When C. arakawae infected with this protozoan sucks blood, sporozoites that enter the body of chicken with the saliva become parasitic on vascular endothelial cells, mainly in the lung, liver, spleen, kidney and such, and form first-generation schizonts. On the fifth to seventh day after infection, first-generation merozoites are released into the blood. These first-generation merozites reenter the vascular endothelial cells distributed throughout the body, grow into second-generation schizonts, leave the host cells and grow in the intercellular space in the late stage. On the 14th day, second-generation merozoites are released into the blood by these schizonts. The merozoites enter erythroid cells and grow, and on the 18th to 19th day, they separate from the host cells and mature as macrogametocytes (female gametocytes) and microgametocytes (male gametocytes).
When C. arakawae sucks blood, both gametocytes are taken up into the midgut together with the blood of the chicken. These gametocytes form macrogametes (female gametes) and microgametes (male gametes), which then conjugate (fertilization) to form zygotes that subsequently form ookinetes which enter the intercellular space in the midgut wall and develop into oocysts after migrating to beneath the outer membrane of the midgut wall. Several tens of sporozoites are formed inside the oocysts, and as the wall of the oocysts breaks down, sporozoites are released into the celom of C. arakawae. They then migrate to the salivary gland to complete the developmental cycle.
Symptoms and lesions in chicken caused by this protozoan infection are observed mainly in the late stages of schizogony when second-generation schizonts grow, and in the gametogony stage when second-generation merozoites become parasitic on erythroid cells and develop into gametocytes. Anemic symptoms and bleeding lesions in various organs and tissues, which are most characteristic of this disease, are thought to be triggered by vascular emboli-accompanying bleeding caused by second-generation schizonts, and disruption of red blood cells due to parasitization by protozoans in the gametogonic stage.
Since C. arakawae, the mediator of the protozoan, inhabits wetlands, this infection has been observed to be prevalent in Southeast Asia, China, North Korea, and South Korea where paddy fields are found. In Japan, since the first observation of leucocytozoonosis, the epidemic has recurred every summer and inflicted a significant loss on poultry productivity. However, the occurrence sharply decreased after feeds started to be supplemented with pyrimethamine (since 1964) and a mixture of pyrimethamine and sulfa drugs (since 1968) as preventive agents. Since then however, acquired resistance of protozoans and residuality issues have restricted the duration of use of effective drugs, and the use of some of these drugs have been banned for safety reasons. Such restrictions are making the prevention of the disease difficult and the damages are also becoming constant issues.
Conventional experimentally produced vaccines against chicken leucocytozoonosis include live vaccines that use sporozoites, inactivated vaccines in which the antigen is a parasite-derived substance, and inactivated vaccines produced out of organ emulsions of protozoan-infected chicken. In the case of live vaccines using sporozoites, chickens are immunized with a small number of sporozoites separated from the salivary glands of C. arakawae [see Non-Patent Document 1]. Examples of inactivated vaccines in which the antigen is a parasite-derived substance are as follows.
Examples of parasite-derived antigens include first-generation schizonts, first-generation merozoites, second-generation schizonts, second-generation merozoites, and soluble antigens derived from the body of chicken and from developing chick embryos. Of these, soluble antigens detected in the serum of infected chicken on the tenth to fifteenth day after sporozoite inoculation have been observed to have nearly the same antigenic characteristics as second-generation schizonts, and a relatively strong immunogenicity similar to that of the second-generation schizonts. Utility has been confirmed for experimental vaccines produced by inactivating such serum-derived antigenic material with formalin [see Non-Patent Document 2].
Furthermore, inactivated vaccines made from organ emulsions were developed to improve the difficulties of obtaining large amounts of the above-described protozoan antigens. The vaccines were experimentally produced out of organ emulsions collected from the thymus, lung, liver, spleen, kidney, and F sac from infected chicken on the thirteenth day after sporozoite inoculation, and then inactivated with formalin. The effects of such vaccines have been confirmed [see Non-Patent Document 3].
To enable a stable and abundant supply of vaccines, the present inventors have provided vaccines that use genetically modified microorganisms made to express immunogenic proteins derived from second-generation schizonts of Leucocytozoon caulleryi [see Patent Document 1].
However, with vaccines developed so far, there is the premise that they will be administered by injection, and is problematic in that their use is limited due to cost and inconvenience.    [Patent Document 1] Granted/Registered Japanese Patent No.3582663    [Non-Patent Document 1] Shiihara et al., “Shizuokaken Yokei Shikenjo Kenkyu Hokoku (Bulletin of Shizuoka Poultry Experiment Station)” 13:25-27 (1978)    [Non-Patent Document 2] Isobe and Suzuki, Jpn, J. Parasitol., 37:214-219 (1988)    [Non-Patent Document 3] Morii, T. et al., J. Parasitol. Res., 76:630-632 (1990)